Convection furnaces are used for the reflowing of solder in the surface mounting of electronic devices to circuit boards. In such furnaces, boards, having preformed solder and/or solder-paste patterns on the boards and devices, travel on a conveyor through a plurality of heating zones. Circulating gas in the furnace is heated by convection heaters above and below the conveyor and impinges on the boards and devices to heat the solder to the reflowing temperature. The gas is typically circulated by fans or gas amplifiers. A cooling section downstream of the heating zones is generally used to cool the boards down from the reflowing temperature.
During solder reflow, an effluent of vaporized flux and solvents is driven from the solder paste. The vaporized flux condenses on surfaces, such as fan blades, air amplifiers, feed lines, or motor shafts, with which the flux vapor comes into contact that are below the flux condensation temperature. Periodic cleaning of the surfaces on which the flux has condensed must be undertaken. Filtration systems have also been used to help remove the flux.
A recent convection furnace design, shown in U.S. Pat. No. 5,347,103, employs gas amplifiers in lieu of fans to circulate air or other gas over the heating elements and to the boards. The gas amplifier introduces a high volume flow of gas into a sealed, pressurizable box. The flow circulates over heating elements in the box to heat the gas, which pressurizes the interior of the box. The heated gas is distributed over a plate having an array of orifices and flows through the orifices to impinge on the product on the conveyor. The gas is recirculated through a return plenum.
Gas amplifiers are based upon the Coanda effect, which describes the phenomenon in which a jet of fluid exiting from a nozzle along a surface tends to follow and adhere to the surface and entrains nearby ambient fluid as it flows. A gas amplifier comprises a body having a fluid flow passage extending therethrough from an entrance to an exit. The entrance to the passage is shaped according to a Coanda profile. A fluid under high pressure (the input) flows radially inwardly into the passage through an annular gap or space at the entrance. The width of the gap is set by a toothed spacer or shim. One surface of the gap joins and forms part of the Coanda profile of the passage through the amplifier. As the high pressure input flow exits the gap, it follows the Coanda profile into the passage and thereby entrains ambient gas (the inflow) through the entrance into its flow.